Carrier communication system



sept. 1o, 1946. f

L. G. ABRAHAM CARRIER COMMUNICATION SYSTEM 2 Sheets-Sheet 1 Filed Aug. 9, 1944 A T TORML'Y /Nl/EA/roR L. G. ABRAHAM I wh..

i lu sept. 1o", 1946.

L. G. ABRAHAM CARRIER comuNmATIoN SYSTEM Filgi Aug. 9, '1944 2 Smets-sheet 2 ATTO/RNE? Patented Sept. 10, 1946 UNITED STATES PATENT `OFFICEy 2,407,238 CARRIER COMMUNICATION SYSTEM `V Leonard G. Abraham, Madison, N. J., assignor to Bell Telephone Laboratories, Incorporated, New Y York, N. YL, a corporation of New York Application August 9, 1944SerialNo. 548,731

A n object of the invention is to adapt such a transmitting and receivingcircuits at the terminal stations orintermediate repeater points as to enable the outgoing voice channel at each terminal or repeater point to be transmitted to the two-wire line `and the incoming voice channel at each terminal to be transmitted from the two-wire line tothe signal reproducing circuit for that channel, over two-wire paths and without intermediate frequency changing operations.

The exact nature of theinvention will be understood from the following detailed description when'read in conjunction with the accompanying drawings-Fig. l of which shows a block schematic of a multichannel, two-way carrier telephone system embodying the invention; and Fig.` `2 of which shows a block schematic cf an intermediate two-way repeater embodying the invention which may be used in the system of Fig. l.

Referring to Fig.` 1 of the drawings, the carrier system shown comprises a west terminal sta tion, to be referred to hereinafter 'as terminal station A, and an east terminal station, to be referred to hereinafter as terminal station B, con` nected by an intermediate main two-wire line ML. which may be an open-wire line, operating in combination to provide four two-way telephone circuits, which may be designated as channels l. 2, 3 and il, between the stations. Each terminal station A and B includes an identical four-channel terminal shown within the. dot-dash boxes labeled TA and TB, respectively, which are respectively associated with four two -wire telephone subscribers lines at the respective stations;

As indicated, channel l of terminal TA at terminal station A, which occupies the voice fre.- quency band, includes a transmitting circuit branch TC`1 and a receiving circuit branch RC1 which are coupled at the left by Vthejhybrid coil H1 and associated balancing network N1 in conjugate relation with eachother and in transmis-v sion relation with the two-wire line Z1 leading to one telephone subscribers transmitting and .receiving telephone equipment y (not shown). The transmitting Vbranch TCI includes the low-pass filter F1 having a cut-off frequency of 2700 cycles, and the receiving branch RCrincludes the lowpass filter F2 having a cut-off frequency of 2700 cycles, and the receiving channel amplifier A.

Channels 2, 3 and 4 of the four-channel terminal at station A are produced by Vshifting by modulation a voice frequencyband to respectively higher frequency positions in the carrier frequency range below a given limiting frequency. Channel 2l comprises a transmitting circuit branch 'ICz and "a receiving branch RCzcoupled at the left by the hybrid coil H2 and associated balancing network N2 in conjugate relation with each other and in transmission relation with the two-Wire line Z2 leading to the transmitting and receiving telephone equipment of a second telephone subscriber. The transmitting branch TG2 includes a low-pass lter F3 having a cut-off `frequency of 2-700 cycles, in its input, followed by a modulator M1 and a 3200 to 5700 cycle band-pass filter F4; and the receivingbra'nchRCz includes the 3200""to 5700 cycle band-pass filter F5 in its input followed by the demodulator DMi, the lowpass filter Fs having a cut-off frequency of 2700 cycles, and the receiving channel ampliiier Ag.

p The modulator M1 in the transmitting branch o TC1 and the demodulator DMI in the receiving branch TCz of'channel No. 2 are supplied with' a carrier frequency of 5900 cycles from the common oscillator O1.

The carrier channel No. 3y comprises a transmitting circuit branch 'ICs and a receiving circuit branch RCs coupled by' the hybrid coil Hi and the associatedbalancing network N3 in conjugate relation with each other and in transmission relation with the two-wire line ls leading 'to the telephone transmitting and receiving equipment of a third telephonesubscrber. The trans mitting branch 'TG3 includes,` the low-pass filter vFt having `a cut-olf frequency of 2700lcycles, in its inputfollowed by themodulator M2 and the 6150A to.18650A cycle band-,pass lilter Fs; and the receiving branch RC3 includes the 6150 to 8650 cycle band-pass filter F94 inits, input followed by the demodulator DlVIz,` the low-pass lter F10 having a cut-ofi frequency of 2700 cycles,` and the receiving channel amp-liner As. The modulator M2 in the, transmittingfbranch TCsjand the demodum latorDMz` in the'receiving branch RC3 of channel No. S'are suppliedv with a carrier frequency of, 885`0icycles'from the common oscillator O2.

Similarly', the transmitting circuit branch T04 and the receiving branch RC4 of channel No. 4 are coupled at the left by the hybrid coil H4 and associated balancing network N4 in conjugate relation with each other and in transmission relation with the two-wire line Zi leading to the telephone transmitting and receiving equipment of a fourth telephone subscriber at terminal station A. The transmitting branch TC4 includes the low-pass filter F11 having a cut-olf frequency of 2700 cycles, in its input followed by a modulator M3 and the 9100 to 11,500 cycle band-pass filter F12. The receiving branch RC4 includes in its input the 9100 to 11,500 cycle band-pass filter F13 followed by the demodulator DMs, the low-pass filter F14 having a cut-oif frequency of 2700 cycles, and the receiving channel amplifier A4. The modulator M3 in the transmitting branch TC4 and the demodulator DMs in the receiving branch RC4 of channel No. 4 are supplied with a carrier frequency of 11,800 cycles from the common oscillator O3.

The four-channel terminal TB at terminal station B includes component circuit elements identical with those described above for the fourchannel terminal TA at terminal Station A as indicated by use of the same identification characters for its corresponding circuit elements eX- cept that each is followed by a prime mark, connected at the right to the four two-wire subscriber lines Z' to ZA as shown.

As the frequencies on the open wire line ML of the carrier channels to be transmitted from terminal station A to terminal station B are different from those on the open wire line ML to be transmitted from terminal station B to terminal station A, the circuits provided for connecting terminal TA to the West end of the two-wire line ML differ from those provided for connecting terminal TB to the east end of the two-wire line ML, as follows.

The four transmitting branches TC1 to T04 of the four-channel terminal TA at terminal station A are connected at the right in parallel to the input of the common transmitting line TLA for the four channels. The common transmitting line TLA, which includes the group transmitting amplifier TAA, has its output connected to the west end of line ML through a low-pass directional fllter DFi having a pass frequency range which includes the frequencies of the voice-channels, and of the three loW frequency carrier channels to be transmitted in the west-to-east direction, and excludes the frequencies of the three higher frequency carrier channels to be transmitted in the east-to-west direction. The four receiving bran-ches RC1 to RC4 of the four-channel terminal TA are connected at the right in parallel to the output of the common receiving line RLA the input of which is connected to the west end of the main line ML through the high-pass directional filter DFz the pass frequency range of which includes the frequencies of the three higher frequency carrier channels to be transmitted in the east-to-west direction, and excludes the frequencies of the three low frequency carrier chan-`- nels to be transmitted in the west-to-east direction, and those of the voice frequency channels. The common receiving line RLA includes between the high-pass directional filter DF2 and the fourchannel terminal TA, reading from right to left, an equalizer EQi, the group demodulator GDM supplied with the group carrier frequency of 20.65 kilocycles from the associated oscillator O4, the low-pass lter F15 having a cut-off at a frequency 4 of 11.8 kllocycles and the receiving group amplifier RAA.

The four transmitting branches TCi' to TCi' of terminal TB at terminal station B are connected at the left in parallel to the input of the common transmitting line TLB including the group modulator GM supplied with a carrier frequency of 20,65 kilocycles from the associated carrier oscillator O5, followed by the group transmitting amplier TAB the output of which is connected through a high-pass directional filter DFa having the same frequency pass range as the high-pass directional filter DFz at the West terminal station A, to the east end of the main line ML. The four receiving branches RC1' to RC4l of terminal TB are connected in parallel at the left to the output of the common receiving line RLB which includes the group receiving amplifier RAB, and the input of the common receiving line RLB is connected to the east end of the main line ML through the low-pass directional lter DFA having the same frequency pass range as the lowpass directional lter DFi at terminal station A.

In addition to the circuits which have been referred to above, in accordance with the invention, terminal station A includes a voice frequency by-pass circuit BCA having its input coupled to the output of the common transmitting line TLA by the hybrid coil H5 and associated line and balancing filter network N5, in conjugate relation with the output of the group transmitting amplier TAA and in transmission relation with the west end of line ML through the lowpass directional filter DFi; and its output coupled by hybrid coil H5 and associated line balancing network N6, in conjugate relation with the output of the group receiving amplifier RAA and in transmission relation with the four parallelconnected receiving branches RC1 to RC4 of the four-channel terminal TA. The by-pass circuit BCA includes in its input the low-pass filter F11 cutting off at a frequency of 3 kilocycles, followed by an equalizer EQ2 and a one-way amplifier A5. Also, a high-pass filter F20 cutting off at a frequency of 3 kilocycles, is inserted in the common receiving line RLA between the output of receiving amplifier RAA and hybrid coil Hs.

Also, in accordance with the invention, termil nal station B includes in addition to the circuits referred to above, the voice frequency by-pass circuit BCB having its input coupled by hybrid coil H7 and associated line and balancing filter network N7, to the input of the common transmitting line TLB in conjugate relation with the input to group modulator GM in that line, and in transmission relation with the four parallelconnected transmitting branches TCi to TC4 of the four-channel terminal Te; and its output coupled by hybrid coil Ha and associated line balancing network Na to the input of the common receiving line RLB in conjugate relation with the input of the group receiving amplifier RAB and in transmission relation with the main line ML through the low-pass directional filter DF3. The by-pass BCB circuit includes the lowpass iilter F1a cutting off at the frequency 3 kilocycles, in its input, followed by the one-Way amplifler A5. Also, a high-pass filter F16 cutting off at a frequency of 3 kilocycles is inserted in the common transmitting line TLB between hybrid coil H7 and the input of group modulator GM.

The channel and group modulators and demodulators used in the circuits described above are preferably of the balanced carrier suppression type, and the oscillators, filters and the Y other transmission apparatus used .may beof any suitable type.' j

The system of Fig. 1 operates as' follows for transmission from the terminal A toy terminal B. The voice frequency signals received over the subscribers line l1 at the terminal station A pass through the hybrid coil H1y to` the low-pass filter F1 inthe transmitting branch `TG1 of channel No. l which restricts 4the frequency bandn to about 2700 cycles and prevents the higher speech frequencies from causing interference in the other three channels. The restricted frequency band in the output of the low-pass filter F1 passes into .the input 'of the ccmmontransmitting line TLA for all four channels.` The voice frequency signals from the 'telephone subscribers associated with the two-wire lines lz, la and Z4,`similarly pass through the hybrid coils H2, Ha and H4, respectively, to the low-pass filters F3, F7 and F11' in the transmitting circuits TCz to TCfi of the respective channels 2 to 4, which restrict them to frequencies below 2700` cycles. The restricted voice band in the outputof lter Fs in the transmitting branch TG2 of channel 2 is combined in the modulator M1 with the carrier frequency; of 5900` cycles suppliedfrom oscillator O1, and the lower sideband 3200 to 57004 cycles of the resulting modulation products is selected by the band-pass filter F4 in the modulator output. The restricted voice frequency band in the output of the 10W- pass` filter F7 in the transmitting branch TCa of channel 3 is combined in the modulator M2 with a carrier frequency of 8850 cycles from oscillator O2, and the lower sideband 6150 to 8650 cycles, is selected by the band-pass filter Fs from the resulting modulation products and is passedy to the common transmitting line TLA of the terminal; The restricted voice frequency bandin the out- Iput of the low-pass filter F11` in the transmitting branch T04 is combined-in` the modulator M3 with the carrier frequency of 11,800 cycles supplied from oscillator O3, and the lower sideband,

9100 to 11,500 cycles, is selected by the band-pass filter F12 from the resulting vmodulation products and is passed tothe commontransmitting line TLA of the terminal; The low-pass filter in ccnjunction with the band-pass filter in yeach transmitting channelv prevents the frequencies of one channel from overlapping those of another channel.

The voice frequency band and the three carrier frequency bands occupying a frequency range b'etween 300 and 11,500 cycles thus superposedin the common transmitting line TLA are amplifiedby the group transmitting amplifier TAA, and the amplified waves are passed out through the hybrid coil H5 and the low-pass directional lter DFi to the main line ML in normal manner, and pass out over that line ML to the terminal station B.

At station B, the frequency band-300-11,500 cycles including the eastwardly directed voice channel and three low frequencycarrier channels incoming over Ythe line ML, is selected by the low-pass directional filter DF4, and passes to the common receiving line RLB of the station in which it will be amplified by the receiving amplifiez` RAB and pass to the parallel-connectednceiving branches of the four-channel `terminal TB.

The low-pass filter F2 in the receiving branch RC1' of channel I will select the frequencies 300` to 2700 cycles of channel I, and reject the frequencies of the other three channels'and `the amplifier A1' will amplify the selected voice frepressed by hybrid jcoil H1 onl the .two-wire line Z1' over which they will be transmitted to the telephone Vreceiving equipment of the east subscriber associated; with channel I. The bandpassY filter 55 in the vreceiving branch RC2l of channel 2 will select the frequencies 3200` to 5700 cycles of channel 2 and will reject the frequencies of the otherthree channels. The selected frequency band, 3200 to 5700 cycles, willbecombined in the demodulator DM1l with the carrier frequency of 5900 cycles supplied from oscillator O4. The low-pass filter Fei in the output of the demodulator DMiI will select the lower sideband, Whichis the original voice frequency band 300 to 2700 cycles supplied tothe transmitting branch cf channel 2 at station A,`from the resulting demodulation products, and will suppress the unwanted sideband demodulation products. The voice frequency signal band selected by lter Fs' will be amplified by the channel receiver amplifier A2', andthe amplified band will be impressed by hybrid Hz on the two-wire line l2'` over which it will be transmitted to the telephone receiving equipment of the east subscriber associated with channel 2.

Similarly, the band-pass filter Fg' in the input of the receiving branch RC3 will select from the impressed waves the frequency band, 6150 to 8650 cycles, of channel 3 and will suppress the frequencies of the other channels. The selected `band will be combined in the demodulator DMz with the carrier frequency of 8850 cycles supplied by the oscillator O2'. The low-pass filter F1o' in the output of the demodulator DM2 will select the lower sideband from the demodulation products, which will be the original voice frequency band supplied to the transmitting branch of channel 3 at terminal station A. The selected voice frequency band will be amplified bythe channel receiving amplifier A3', and the amplified frequency band impressed by hybrid coil H3 on the two-wire line Z3 leading to the telephone receiving equipment of the east subscriber associated with channel 3. The band filter F13' in the receiving branch RC4 of the four-channel terminal CTB will select the frequency band, 9100 to 11500 cycles, of channel 4 from the impressed waves, and will reject the frequencies of the other three channels. The selected band, 9100 to 11500 cycles, will `be combined inv the modulator DMa' with the carrier frequency of 11800 cycles from the oscillator Oa. The low-pass filtery F14 in the output of that demodulator will select from the demodulation products the lower sideband, which will be the original voice frequency band supplied to the transmitting branch of channel 4 at terminal station A. The selected voice frequency band will be amplified in the amplier A4' and will be impressed by the hybrid coil H4 on the two-wire line Z4' overt which it will be transmitted to the telephone receiving equipment of the eastnsubscriber associated with channel 4.

The operation of the system of Fig. 1 for signal transmission in the opposite direction from terminal B to terminal A is as follows:

The voice frequency signals received over the two-Wire lines li-, Z2', la' and Z4 from the tele` phone transmitting equipment of the four associated telephone subscribers are impressed by the hybrid coils Hl to H4', respectively, on the transmitting branches TC1 to T04, respectively, of channels Ito 4.

The speech frequencies impressed on `transmitting channel` TG1 will pass to the low-pass ilter Fr which will restrict the frequency band to frequencies below about 2700 cycles, and prevent the higher speech frequencies from causing interference in the other channels. The restricted frequency band in the output of the low-pass filter F1 will pass into the common transmitting line TLB.

The voice frequency signal band impressed on the transmitting channel TG2 of the terminal TB will be restricted by the low-pass filter F3 to the frequency range below 2700 cycles, and the restricted frequency band will be combined in the modulator M1' with the carrier frequency of 5900 cycles supplied by oscillator O1' to produce modulation products of which the lower sideband, 3200 to 5700 cycles, will be selected by the bandpass filter F4. The selected band will be transmitted. to the co-mmon transmitting line TLB of terminal station B. Similarly, the voice frequency signals received by the transmitting circuits TCa' and T04 of the four-channel terminal TB from the east subscribers associated with the two-wire lines la and Z4', respectively, will be restricted to frequencies below 2700 cycles by the low-pass filters F2 and F11 in the respective branches TG3' and TC4' and will be combined with the carrier frequencies of 8850 cycles and 11,800 cycles, respectively, in the modulators M3 and Mi of channels 3 and il, respectively, to produce modulation products. The lower sidebands 6150 to 8550 cycles and 9100 to 11,500 cycles of the mod ulation products in the outputs f modulators Ma and M4', respectively, will be selected by the band-pass filters Fa and F12', respectively, and will be transmitted to the common transmitting line TLB for all four channels.

One energy portion of the input band of frequencies ranging from 300 to 11,500 cycles passing through hybrid coil H7 will be diverted to the outgoing portion of the common transmitting TLB, in which it will be restricted to frequencies above 3 kilocycles by high-pass filter F10. The restricted frequency band comprising the three low frequency carrier channels from terminal TB, will be combined in the group modulator GM with the carrier frequency of 20.65 kilocycles from the associated carrier oscillator O5 to translate them to the higher carrier frequency range for the east-towest direction of transmission. The products of modulation will be amplified by the transmitting amplifier TAB and supplied to the high-pass directional filter DFa which will pass the upper side-band comprising frequencies in the range 20.95 to 32.15 kilocycles to the line ML, and suppress the lower frequencies.

The other energy portion of theinput' frequency band of 300 to 11,500 kilocycles passing through hybrid coil H7 will be diverted into the by-pass circuit BCB. The 3-kilocycle low-pass filter Fia in that circuit will select the voice channel and suppress the three low frequency carrier channels. The selected voice frequency channel will be amplified by the amplifier A0 in by-pass circuit BCB, and the amplified voice frequencies will pass through the hybrid coil H0 in the common receiving line RLB of the terminal to the low-pass directional filter DF4 which will transmit it to the line ML over which it will be transmitted, along with the carrier frequency band 20.95 to 32.15 kilocycles received throughthe high-pass directional filter DFs, to the west terminal station A.

At terminal station A, the three carrier frequency channels in the frequecy range 20.95 to 32.15 kilocycles incoming over the line ML will be selected' by the high pass directional filter DFz and will pass through the high frequency equalizer EQ1 to the group demodulator GDM in the common receiving line RLA, in which they will be combined as a group with the carrier frequency of 20.65 kilocycles from the associated oscillator O4 to produce modulation products of which the lower side-band within the frequency range 0 to 11.8 kilocycles is selected by the lowpass filter F15. The selected frequency band comprising the three incoming carrier channels reduced to the frequency range which they had at the output of the four-channel terminal TB at terminal station B, will be amplified by the receiving amplifier RAA and passed to the highpass filter F20 cutting olf at 3 kilocycles, which will operate to suppress the voice frequencies and transmit the carrier frequencies to the receiving circuits of the four-channel terminal TA at terminal A.

The voice frequency channel received at terminal station A over the line ML along with the three high frequency carrier channels, will be selected by the low-pass directional filter DF1 at that station and will pass through the hybrid coil H5 into the by-pass circuit BCA in which it will be selected by the low-pass filter F17 cutting off at 3 kilocycles. The selected voice channel will pass through equalizer EQ2 (which is a voice frequency equalizer to take care of the frequency characteristics of the preceding line, and then through the amplifier A5 which for this purpose should be a low power output amplifier of moderate gain. The amplified voice frequency signals in the output of amplifier A5 will pass through hybrid coil H0 to the output portion of the common receiving line RLA and will be passed over that circuit along with the frequencies of the three incoming carrier channels reduced to the same frequencies as the outgoing carrier channels received through that hybrid coil from the high-pass filter F20, to the parallelconnected channel receiving branches of the f our-channel terminal TA.

The high-pass filter F20 cutting off at 3 kilocycles in the common receiving line RLA in front of hybrid coil H0, is provided so that if the same speech currents weie impressed from the distant end upon the 20.65 to 32.45-kilocycle line frequencies, there would be no interference between the two transmission paths carrying the same speech. In some cases this blocking highpass filter F20 might be omitted at a terminal and improved transmission for the over-all circuit obtained. For example, with 6 decibels loss over-all for the circuit by carrier means, allowing one direction of two-wire voire frequency transmission from an intermediate point to a terminal to be down 10 decibels by reducing the gain of the amplifier A5 in the by-pass circuit BCA by 10 decibels, would make transmission from one terminal to the other via the voice path substantially negligible for most purposes. The same expedient at an intermediate repeater point would make the loss greater from an intermediate point to a terminal or to another intermediate point into the terminal beyond the repeater, but would allow normal low loss from terminal to terminal without interference.

The incoming voice frequency channel impressed on the four-channel terminal TA will be selected by the low pass filter F2 in the receiving branch RC1 of channel I and, after amplification by the amplifier A1 therein, will be impressed by the hybrid coil H1 on the two-wire line l1 over which it will be transmitted to the telephone receiving equipment of the West subscriber associated with channel I. The three carrier channels of frequencies 3200 to 5700 cycles, 6150 to 8650 cycles and 9100 to 11500 cycles, respectively, impressed from the out-put of the common receiving line RLA on the receiving circuits RC2 to RC4 of channels 2, 3 and 4 of the four-channel terminal CTA in a manner similar to that pre/- viously described for the similar four-terminal TB at terminal station B for transmission in the opposite direction will be separated from each other and demodulated to voice frequencies for transmission over the associated two-wire lines Z2, Za and Z4 to the telephone receiving equipment of the respective subscribers associated with each channel.

Fig. 2 shows the circuit of a two-way repeater which may be inserted in the two-Way line ML at a station C intermediate the terminal stations A and B of the system of Fig. 1, to provide a desired amount of amplification of the three carrier communication channels and one voice and communication channel transmitted in each direction over the system.

The two-way repeater at'station C includes a low-pass directional filter DF5 and a high-pass directional filter DFG connected in parallel to the west section of line ML leading to station A, and another low-pass directional filter DFv and another high-pass directional filter DFS connected in parallel to the east section of line ML leadingto station B. The low-pass filters Dls and DFv have the same frequency pass range, 300V to 11,500 cycles, as the low-pass directional filters, DF1 and DFi at terminal stations A and B, respectively, which includes the frequencies of the voice channels transmitted in both directions and of the three lower frequency carrier channels transmitted in the west-to-east direction and excludes the frequencies of the three higher frequency carrier communication channels transmitted in the east-tc-west direction over the system. The high-pass filters DFG and DFS have the same frequency pass range, 20.95 to 32.15 kilocycles, as the high-pass `directional filters DFz and DF3 at terminal stations A and B, respectively, which includes the frequencies of the three higher frequency carrier channels transmitted in the east-to-west direction over the system and excludes the frequencies of the Voice channels transmitted in both directions' and of the three lower frequency carrier channels transmitted in the west-toeast direction over the system. l The high-pass filters DFat and DFS are Vcon-- nected to each other through `the one-way repeating path WAi including the high frequency equalizer EQa and the one-way amplifier A7 pointed in the east-to-west direction. The path WA1, therefore, serves to repeat in amplifled form the three east-to-west `carrier com-` munication channels in the frequency range 20.95 to 32.15 kilocycles from the east to the West section of the two-way line ML.

The west section of the two-way line ML is connected through low-pass directional lter DF5 and hybrid coil .H9 to the input of thev oneway repeating path EA including the low fre-` quency equalizer EQ4 and the y Aa pointed in the west-to-east direction, and to the output of the one-way repeating path. WAL: including the low-pass filter Fmcutting off at `a frequency of 3 kilocycles in" its `input and the following low frequency equalizer EQs and oneone-way amplifierv Y way amplifier-A9 pointed inthe east-to-west-uirection. The output of the-repeating path EA and the input of the repeating path WAz are connected through the hybrid coil Hip and the lowpass directional `filter DF? to the east section of thejtwo-wayline ML. The hybrid -coil Heand its associated' line and balancing filter network N9 serve to couple the input of the `repeating path EA and the output ofthe repeating path WAz in conjugate relation with each other and in transmission relationwith the west section of line ML through filter DFs; Similarly the hybrid coil H10 and its associated line and balancing filter network N10 serve to couple the output of repeating path EA and the input of the repeating path WAz in conjugate relation with each other and in transmission relation with the east section of line ML through filter DF7.

Thus, the voice channel andthe three -loWer frequency carrier channels in the west-to-east direction occupying the frequency range 300 to 11,500 cycles received at therrepeater station C over the west section of line ML will pass through the 10W-pass directional lter DFs and hybrid coil H9 to repeating path EA in which they 'will 4be equalized by the common equalizer EQ4 and amplied to the desired levelin common amplilier As. The amplifier voice channel and lower frequency carrier channels in the output ofamplier As will pass through the hybrid coil H10 and low-pass` directional filter DFv to theeast section of the two-way line ML over which they will be transmitted to terminalstationB. v

The voice channel in the east-to-west'direcf tion received at the repeater station C over the east section of line ML from terminal station B, will pass through the low-pass directional filter DF7 and hybrid coil I-Iro to the repeating Apath WAz in which it will be selected by the S-kilocycle low-pass4 filter F19, equalized by low frequency equalizer EQs and amplified to the desired level by amplifier Ag. The amplified voice channel in the output of amplifier A9 will pass through hybrid coil H9 and low-pass directional filter DFs to the west section of line ML over which it will be transmitted to the West terminal station A along with the" amplied carrier channels in the frequency range 20.95 to 32.15 kilo.

cycles received through high-pass directional filter DFS. j

Thus, it willbe seen that the effect of the added by-pass circuitsY and filters at terminal stations A and B and repeater station C in the multi-channel carrier ysystem of Figs. l `and 2 is to adapt the system for transmitting voice channels in both directions on a two-Wire basis between the subscribers lines at the terminal stations and 'from there to any intermediate point on the two-wire line. When filters F17 and F1a are omitted, over-all transmission is on a carrier basis but transmissionlto intermediate points is on a voice frequency basis in both directions.

Various modicationsof the circuits illustrated and described which are within the spirit' and scope of the invention will occur to persons skilled in the. art. y

What is claimed is:

1. In a multiplex communication system, two terminal stations, a two-way, two-wire lineconnecting saidstations,` means at one station for providing a plurality of low frequency carrier communication channels Vfor transmission to the other station,`me ans at said other station for providing a plurality of` higher frequency carrier y communication channels for transmission to lsaid,

one station, a 10W-pass directional filtering means and av high-pass directional filter means at saidone station for respectively supplying the outgoing low frequency carrier channels to said line and receiving the incoming higher frequency carrier channels from said line, another low-pass directional filtering means and another high-pass directional filtering means at said other station for respectively selecting the incoming low frequency carrier channels from said line and sup) plying Athe outgoing higher frequency carrier channels to said line, means at each station for separately supplying an outgoing Voice frequency communication channel through the low-pass filtering means thereat to said line for transmission thereover, and means at each station connected to the drop side of the low-pass directional iiltering means at that station, for selectively receiving the voice frequency channel incoming over said line.

2. In a multiplex communication station, terminal stations, a two-way, two-Wire line connecting said stations, means at one Station for providing a voice frequency communication channel` and a plurality of low frequency carrier communication Vchannels for transmission to the other station, means at said other station for providing another voice communication frequency channel and a plurality of higher frequency carrier communication channels for transmission to said one station, a low-pass directional hltering means and a high-pass directional filtering means respectively having a pass frequency range including the frequencies of the voice channels and said low frequency carrier channels, and of the higher frequency carrier channels, connected to the two- Wire line at each terminal station, a common transmitting circuit including the low-pass directional filtering means at said one station for transmitting the first voice channel and said loW frcquency carrier channels to said line, a common transmitting circuit includingthe high-pass directional filtering means at said other station for transmitting said higher frequencyl carrier channels -to said line, a common receiving circuit including the high-pass directional filtering means at said one station for selectively receiving the incoming higher frequency carrier channels from said line, a common receiving circuit including the low-pass directional filtering means at said other station for selectively receiving the voice frequency channel and plurality of low frequency carrier channels incoming over said line, means at said other station for impressing the outgoing voicechannel on said line through the low-pass directional filtering means in the common receiving circuit of that station, and means at said one station for selectively receiving the voice channel incoming over said line through the low-pass directional filtering means in the common transmitting circuit of that station.

3. In a multiplex communication system, terminal stations, a two-Way, two-Wire line connecting said stations, means at one station for providing a Vvoice frequency communication chan-- nel and a plurality of low frequency carrier channels for transmission to the other station, means at said other station for providing'another voice frequency communication channel and a plurality of higher frequency communication channels for transmission tov said one station, means at said other station for impressing the outgoing voice channel and said plurality of higher frequency carrier channels on said two-Wire line for transmission thereoverVand for separately receiving the incoming voice channel and low frequency carrier channels from said line, a low-pass direc'- tional filtering means and a high-pass directional filtering means having pass frequency ranges respectively including those of said voice channels and 10W frequency carrier channels, and those of said higher frequency carrier channels, connected to said two-Wire line at said one station, means for impressing the outgoing voice frequency channel and low frequency carrier channels through said low-pass filtering means on said twowire line at said one station, and means for selectively receiving the voice channel and higher frequency carrier channels incoming over said line at said one station from the drop sides of the low-pass ltering means and the high-pass filtering means, respectively, at that station.

4. In a multiplex communication system, terminal stations, a two-Way, two-Wire line connecting said stations, means at one station for impressing a voice frequency communication channel and a plurality of high frequency carrier communication channels on said two-wire line for transmission thereover to the other station, a common transmitting circuit at said other station, including a low-pass directional ltering means of suitable pass frequency range in its output for impressing another voice channel and a plurality of low frequency carrier channels on said two-Wire line for transmission thereover to said one sta-- tion, a common receiving circuit at said other station, including a high-pass directional lterin-g means of suitable pass frequency range in its input for selectively receiving the higher frequency carrier channels incoming over said line and means for demodulating the selected higher frequency carrier channels to desired lower frequencies, means connected to the drop side of said low-pass directional filtering means at said other station for selectively receiving the voice channel incoming over said two-Wire line from said one station, and separate receiving means at said other station for the selected incoming voice channel and demodulated higher frequency carrier channels.

5. In a multiplex communication system employing a group of low frequency channels including a voice frequency channel and a plurality of low frequency carrier channels for signal transmission in one direction and a second group of channels including a voice frequency channel and a plurality of higher frequency carrier channels for signal transmission in the opposite direction, a two-Wire line for transmitting all of the channels between terminals of said system, means at one terminal for generating the voice frequency channel and the low frequency carrier channels for said one direction, a transmitting circuit at said one terminal including a low-pass directional filter for transmitting the generated group of low frequency channels to said line, a receiving circuit at said one terminal, including a high-pass directional filter in its input for selecting the grou-p of higher frequency carrier channels incoming over said line and means for demodulating the selected channels as a group to desired lower frequencies, a circuit at said one terminal for selectively receiving the voice channel incoming over said line, having its input connected to thc drop side of said low-pass directional filter in said transmitting circuit, and its output connected to said receiving circuit in the output of said demodulating means, and means connected to the output of said receiving circuit for separating and detecting the communication signalsl from 13 the incoming voice channel and demodulated carrier channels therein.

6. The system of claim 5, in which the input of said circuit at said one terminal for selectively receiving the voice channel incoming over said line is coupled to said transmitting circuit in conjugate relation with the input thereof and in transmission relation with said line through said low-pass directional filter, and the output of the incoming voice channel selecting circuit is connected to said receiving circuit in conjugate relation with the output of said demodulating means and in transmission relation with said channel separating and signal detecting means.

7. The system of claim in which a hybrid coil and associated line balancing network connects the incoming voice channel selecting circuit at said one terminal in conjugate relation with the input of said transmitting circuit and in transmission relation with said line through said low-pass directional filter, another hybrid coil and associated line balancing network connects the output of said incoming voice channel selecting circuit to said receiving circuit in conjugate relation with the output of said demodulating means therein and in transmission relation with said channel separating and signal detecting means, and filtering means adapted to suppress voice frequencies and to transmit higher frequencies is inserted in said' receiving circuit between said demodulating means and the latter hybrid coil.

8. In a multiplex communication system, terminal stations, a two-way, two-Wire line connecting said stations, a source of low frequency carrier communication channels and a voice frequency communication channel at each station, low-pass filtering and high-pass filtering means connected to said line at each station, a common channel transmitting circuit and a common channel receiving circuit at each station, the common transmitting circuit at one station being supplied with the low frequency carrier channels and the voice channel generated by said source thereat and including the low-pass filtering means at the station, the common transmitting circuit at the other station being supplied with the low frequency carrier channels and voice channel generated by said source thereat, and including modulating means for stepping up the supplied low frequency carrier channels to desired higher frequencies for transmission over said line to said one station, and the high-pass filtering means at the station, said low-pass and high-pass filtering means at the stations being respectively adapted for selectively transmitting the frequencies of the voice and low frequency carrier channels, and the frequencies of the stepped-up higher frequency carrier channels, the common channel receiving circuit at said other station including the low-pass filtering means thereat for selectively receiving the voice channel and low frequency carrier channels received over said line, the, common channel re-` ceiving circuit of said one station including the high-pass filtering means thereat for selectively receiving the higher frequency carrier channels incoming over said line and demodulating means for translating the frequencies of the` selected 14 higher frequency carrier channels to their original low frequencies, a circuit at said other station selective to the frequencies of the outgoing voice frequency channel, having its input coupled to the common transmitting circuit at that station in front of the modulating means therein so as to be supplied with that channel from said source at the station, and having its output coupled to the input of the common receiving circuit at the station on the drop side of the lowpass filtering means therein, another circuit selective to the frequencies of the incoming voice channel having its input coupled to the output of the common transmitting circuit at said one station on the drop side of the low-pass filtering means therein, and its output connected to the common receiving circuit of that station beyond the demodulating means therein, and means connected to the output of the common receiving circuit at each station to separate the received voice and carrier channels therein and to detect the communication signals therefrom.

9. The system of claim 8 in which the circuit selective to the frequencies of the voice frequency channel at each station is connected to the common channel transmitting and receiving circuits at each terminal by hybrid coils and associated line balancing networks.

10. In a multiplex communication system, terminal stations, a two-way, two-Wire line connecting said stations, means to supply to said line at one terminal station one voice frequency cornmunication channel and a plurality of low frequency Y carrier communication channels above the voice frequency range, for transmission in one direction tothe other terminal station, means to supply to said line at the other terminal station another voice frequency communication channel and a plurality of higher frequency carrier communication channels, for transmission in the opposite direction to said one station, and a two-way repeater connecting sections of said two-Way lineat a point intermediate said terminal stations, said repeater including low-pass filtering means and high-pass filtering means for respectively selectively transmitting the frequencies of the voice channels and said low frequency carrier channels and of said higher frequency rcarrier channels, connected to each of said two-way line sections, a one-way amplifying path including the high-pass filtering means connected to both of said line sections, for selectively repeating said higher frequency carrier channels in said opposite direction between said line sections, a second one-way amplifying path including the low-pass filtering means connected to both said line sections for selectively repeating one voice channel and said plurality of low frequency carrier channels in said one direction between said line sections and a third one-way amplifying path selective to voice frequencies, connected between the dropsides of the low-pass filtering means connected to the two-line sections, and in conjugate relation with said second amplifying path, for selectively repeating the voice channel in said opposite direction between said line sections.

LEONARD G. ABRAHAM. 

